1. Field of the Invention
The present invention relates to the screening of drugs, especially random peptides, in yeast cells for the ability to interact with proteins involved in the post-translational modification, transport of and response to yeast pheromones or substitutes therefor.
2. Description of the Background Art
Drug Screening
The identification of biological activity in new molecules has historically been accomplished through the use of in vitro assays or whole animals. Intact biological entities, either cells or whole organisms, have been used to screen for anti-bacterial, anti-fungal, anti-parasitic and anti-viral agents in vitro. Cultured mammalian cells have also been used in screens designed to detect potential therapeutic compounds. A variety of bioassay endpoints are exploited in mammalian cell screens including the stimulation of growth or differentiation of cells, changes in cell motility, the production of particular metabolites, the expression of specific proteins within cells, altered protein function, and altered conductance properties. Cytotoxic compounds used in cancer chemotherapy have been identified through their ability to inhibit the growth of tumor cells in vitro and in vivo. In addition to cultures of dispersed cells, whole tissues have served in bioassays, as in those based on the contractility of muscle.
In vitro testing is a preferred methodology in that it permits the design of high-throughput screens: small quantities of large numbers of compounds can be tested in a short period of time and at low expense. Optimally, animals are reserved for the latter stages of compound evaluation and are not used in the discovery phase; the use of whole animals is labor-intensive and extremely expensive.
Microorganisms, to a much greater extent than mammalian cells and tissues, can be easily exploited for use in rapid drug screens. Yeast provide a particularly attractive test system; extensive analysis of this organism has revealed the conservation of structure and function of a variety of proteins active in basic cellular processes in both yeast and higher eukaryotes.
The search for agonists and antagonists of cellular receptors has been an intense area of research aimed at drug discovery due to the elegant specificity of these molecular targets. Drug screening has been carried out using whole cells expressing functional receptors and, recently, binding assays employing membrane fractions or purified receptors have been designed to screen compound libraries for competitive ligands. Duke University, WO92/05244 (Apr. 2, 1992) describes the expression of mammalian G protein-coupled receptors in yeast and a means of identifying agonists and antagonists of those receptors using that organism.
In addition, yeast are, of course, used in the discovery of antifungal compounds; Etienne et al. (1990) describe the use of Saccharomyces cerevisiae mutant strains made highly sensitive to a large range of antibiotics, for the rapid detection of antifungals.
Yeast Pheromone System Proteins and Their Metabolic Function
Haploid yeast cells are able not only to grow vegetatively, but also to mate to form a diploid cell. The two mating types (“sexes”) of haploid cells are designated a and α. The a cells produce the dodecapeptide a-factor, and the α cells, the tridecapeptide α-factor. Because a-factor and α-factor elicit a mating response in the yeast cell of the opposite “sex”, they are called “pheromones”. These pheromones, as well as other proteins specifically involved in the production or transport of, or response to, pheromones, are considered “pheromone system proteins”.
The gene encoding a-factor pheromone, like the a-factor receptor gene, is an a cell-specific gene: a cell-specific genes are only expressed in a cells. The gene encoding α-factor pheromone, like the a-factor receptor gene, is an α cell-specific gene: α cell-specific genes are only expressed in α cells. Other yeast genes belong to a haploid-specific gene set and are expressed in haploid cells (a cells or α cells) but not in diploid (a/α) cells. In addition, there exists a diploid cell-specific gene set, including those genes involved in sporulation.
In eukaryotic cells, RNA polymerase II promoters contain a specific sequence (the TATA box) to which the transcription factor TFIID (TATA binding protein or TBP) binds. An active transcription initiation complex includes TFIID, accessory initiation proteins, and RNA Pol II. As in higher eukaryotic cells, the TATA box is an essential control sequence in yeast promoters. Yeast TATA-box-binding protein (TBP) was identified by its ability to substitute in function for mammalian TFIID [Buratowski et al., Nature 334, 37 (1988); Cavallini et al., Nature 334, 77 (1988)]. With only a few apparent exceptions [transcription of some glycolytic enzyme genes, see Struhl, Mol. Cell. Biol. 6, 3847 (1986) and Ogden et al., Mol. Cell Biol. 6, 4335 (1986)] transcription of yeast genes requires the proximal TATA box element and TFIID binding for initiation of transcription. Also required for efficient transcription are gene-specific activator proteins; the precise mechanism whereby these gene-specific regulatory proteins influence transcription has not been completely elucidated.
MCM1p (encoded in the MCM1 gene) is a non-cell-type-specific transcription factor in yeast. MCM1p acts alone or in concert with other regulatory proteins to control expression of a- and α-cell specific genes. Yeast mating type loci encode the regulator proteins that contribute to the control of cell type-specific expression. These proteins are Mata1p (encoded by the MATa gene) and Matα1p and Matα2p (encoded by the MATα locus). MCM1p activates transcription of a-specific genes by binding to an upstream activation sequence (UAS) located in the control region of a-specific genes. Matα1p and MCM1p interact to enhance each other's binding to specific UAS binding sites to activate α-cell-specific gene transcription in α-cells. Matα2p associates with MCM1p to repress a-specific gene transcription in α-cells. In diploid (a/α) cells. Matα1p and Matα2p associate to repress the transcription of haploid-specific genes. The Matα1p/Matα2p regulatory entity is found only in diploid cells.
Yeast contain two genes encoding the α-factor pheromone. MFα1 and MFα2. Analysis of yeast bearing mutations in these sequences indicates that MFα1 gives rise to the majority of α-factor produced by cells. Expression occurs at a higher level from MFα1 than from MFα2 (Kurjan, Mol. Cell. Biol. 5, 787 (1985).
The MFα1 gene of yeast encodes a 165 aa precursor protein containing an 85 aa leader sequence at the N-terminus. The leader includes a 19 aa signal sequence and a 66 aa sequence which contains sites for the addition of three oligosaccharide side chains (Kurjan and Herskowitz, Cell 39, 933 (1982); Singh et al. Nuc. Acids Res. 11, 4049 (1983); Julius et al. Cell 36, 309 (1984). Four tandem copies of the 13 aa α-factor are present in the C-terminal portion of the precursor; 6-8 aa spacer peptides precede the α-factor sequences (see FIG. 2).
After translocation of the nascent α-factor polypeptide to the ER, the signal sequence is cleaved from the precursor protein to yield pro-α-factor (Waters et al. J. Biol. Chem. 263, 6209 (1988). The core N-linked carbohydrate is added to three sites in the N-terminus of pro-α-factor (Emter et al. Biochem. Biophys. Res. Commun. 116, 822 (1983); Julius et al. Cell 36, 309 (1984); Julius et al. Cell 37, 1075 (1984). Additional glycosylation occurs in the Golgi prior to cleavage of pro-α-factor by the KEX2 endopeptidase. This enzyme cleaves within each of the spacer repeats leaving a Lys-Arg sequence attached to the C-terminus of α-factor peptide (Julius et al. Cell 37, 1075 (1984). The Lys-Arg sequence is removed by the action of the KEX-1 protease (Dmochowska et al. Cell 50, 573 (1987). The additional spacer residues present at the N-terminus of α-factor peptide are removed by the dipeptidyl aminopeptidase encoded by STE13 (Julius et al. Cell 32, 839 (1983). Four α-factor peptides are released from each precursor protein via the proteolytic processing outlined above and the mature α-factor is secreted from the cell.
Precursors of the 12 aa mature a-factor peptide are encoded in the MFa1 and MFa2 genes and are 36 aa and 38 aa residues, respectively (for schematic of MFa1 gene see FIG. 5). The precursors contain one copy of a-factor and the products of the two genes differ in sequence at one amino acid. The two forms of a-factor are produced in equal amounts by a cells (Manney et al. in Sexual interactions in eukaryotic microbes, p21, Academic Press, New York (1981).
Processing of a-factor entails a process that differs in every detail from that of α-factor. The processing of a-factor begins in the cytosol and involves the farnesylation of the C-terminal cysteine residue near the carboxyl terminus (-CVIA) by a farnesyl transferase (Schafer et al. Science 245, 379 (1989); Schafer et al. Science 249, 1133 (1990). The α and β subunits of the farnesyl transferase are encoded by the RAM2 and RAM1 genes, respectively (He et al. Proc. Natl. Acad. Sci. 88, 11373 (1991). Subsequent to farnesylation is the proteolytic removal of the three amino acids that are C-terminal to the modified cysteine by a membrane-bound endoprotease. Next, the carboxy-terminal farnesylated cysteine residue is modified further: the carboxyl group is methylated by the product of the STE14 gene. STE14p is a membrane-bound S-farnesyl-cysteine carboxyl methyl transferase (Hrycyna et al. EMBO. J. 10, 1699 (1991). The mechanisms of the N-terminal processing of a-factor have not been elucidated. After processing of the precursors is complete, mature a-factor is transported to the extracellular space by the product of the STE6 gene (Kuchler et. al. EMBO J. 8, 3973 (1989), an ATP-binding cassette (ABC) transporter.
In normal S. cerevisiae (budding yeast) a cells, the α-factor binds the G protein-coupled membrane receptor STE2. The G protein dissociates into the Gα and Gβγ subunits, and the Gβγ binds an unidentified effector, which in turn activates a number of genes. STE20, a kinase, activates STE5, a protein of unknown function. STE5 activates STE11 kinase, which stimulates STE7 kinase, which induces the KSS1 and/or FUS3 kinases. These switch on expression of the transcription factor STE12. STE12 stimulates expression of a wide variety of genes involved in mating, including FUS1 (cell fusion), FAR1 (cell-cycle arrest), STE2 (the receptor), MFA1 (the pheromone), SST2 (recovery), KAR3 (nuclear fusion) and STE6 (pheromone secretion). Other genes activated by the pathway are CHS1, AGα1, and KAR3. The multiply tandem sequence TGAAACA has been recognized as a “pheromone response element” found in the 5′-flanking regions of many of the genes of this pathway.
One of the responses to mating pheromone is the transient arrest of the yeast cell in the G1 phase of the cell cycle. This requires that all three G1 cyclins (CLN1, CLN2, CLN3) be inactivated. It is believed that FUS3 inactivates CLN3, and FAR1 inhibits CLN2. (The product responsible for inactivating CLN1 is unknown).
The growth arrest is terminated by a number of different mechanisms. First, the α-factor receptor is internalized following binding of the pheromone, resulting in a transient decrease in the number of pheromone binding sites. Second, the C-terminal tail of the receptor is phosphorylated consequent to ligand binding, resulting in uncoupling of the receptor from the transducing G proteins. Third, pheromone-induced increases in expression of GPA1p (the Gα-subunit of the heterotrimeric G protein) increase the level of the α subunit relative to the Gβ and Gγ subunits, resulting in reduction in the level of free Gβγ and consequent inactivation of the pheromone response pathway. Additional mechanisms include induction of the expression of SST2 and BAR1 and phosphorylation of the α subunit (perhaps by SVG1).
Signaling is inhibited by expression of a number of genes, including CDC36, CDC39, CDC72, CDC73, and SRM1. Inactivation of these genes leads to activation of the signaling pathway.
A similar pheromone signaling pathway may be discerned in α cells, but the nomenclature is different in some cases (e.g., STE3 instead of STE2).
Other yeast also have G protein-mediated mating factor response pathways. For example, in the fission yeast S. pombe, the M factor binds the MAP3 receptor, or the P-factor the MAM2 receptor. The dissociation of the G protein activates a kinase cascade (BYR2, BYR1, SPK1), which in turn stimulates a transcription factor (STE11). However, in S. pombe, the Gα subunit transmits the signal, and there are of course other differences in detail.
Pheromone Pathway Mutants
The effects of spontaneous and induced mutations in pheromone pathway genes have been studied. These include the α-factor (MFα1 and MFα2) genes, see Kurjan, Mol. Cell. Biol., 5:787 (1985); the a-factor (MFa1 and MFa2) genes, see Michaelis and Herskowitz, Mol. Cell. Biol. 8:1309 (1988); the pheromone receptor (STE2 and STE3) genes, see Mackay and Manney, Genetics, 76:273 (1974), Hartwell, J. Cell. Biol., 85:811 (1980), Hagen, et al., P.N.A.S. (USA). 83:1418 (1986); the FAR1 gene, see Chang and Herskowitz, Cell, 63:999 (1990); and the SST2 gene, see Chan and Otte, Mol. Cell. Biol., 2:11 (1982).
Expression of Foreign Proteins in Yeast Cells
A wide variety of foreign proteins have been produced in S. cerevisiae, either solely in the yeast cytoplasm or through exploitation of the yeast secretory pathway (Kingsman et al. TIBTECH 5, 53 (1987). These proteins include, as examples, insulin-like growth factor receptor (Steube et al. Eur. J. Biochem. 198, 651 (1991), influenza virus hemagglutinin (Jabbar et al. Proc. Natl. Acad. Sci. 82, 2019 (1985), rat liver cytochrome P-450 (Oeda et al. DNA 4, 203 (1985) and functional mammalian antibodies (Wood et al. Nature 314, 446 (1985). Use of the yeast secretory pathway is preferred since it increases the likelihood of achieving faithful folding, glycosylation and stability of the foreign protein. Thus, expression of heterologous proteins in yeast often involves fusion of the signal sequences encoded in the genes of yeast secretory proteins (e.g., α-factor pheromone or the SUC2 [invertase] gene) to the coding region of foreign protein genes.
A number of yeast expression vectors have been designed to permit the constitutive or regulated expression of foreign proteins. Constitutive promoters are derived from highly expressed genes such as those encoding metabolic enzymes like phosphoglycerate kinase (PGK1) or alcohol dehydrogenase I (ADH1) and regulatable promoters have been derived from a number of genes including the galactokinase (GAL1) gene. In addition, supersecreting yeast mutants can be derived; these strains secrete mammalian proteins more efficiently and are used as “production” strains to generate large quantities of biologically active mammalian proteins in yeast (Moir and Davidow, Meth. in Enzymol. 194, 491 (1991).
A variety of heterologous proteins have been expressed in yeast cells as a means of generating the quantity of protein required for commercial use or for biochemical study (Kingsman et al. TIBTECH 5, 53 (1987). In addition, a number of mammalian proteins have been expressed in yeast in order to determine whether the proteins (1) will functionally substitute for cognate proteins normally expressed within that organism or (2) will interact with accessory yeast proteins to accomplish a specific function. Thus it has been determined that a human TBP with altered binding specificity will function to initiate transcription in yeast [Strubin and Struhl, Cell 68, 721 (1992)]. In addition, mammalian steroid hormone receptors [Metzger et al. (1988); Schena and Yamamoto (1988)] and human p53 [Schärer and Iggo, Nuc. Acids Res. 20, 1539 (1992)] were shown to activate transcription in yeast.
Expression in yeast of the gag-pol gene of HIV-1 results in the processing of the gag protein precursor to yield the products which normally arise within the virion; processing in yeast, as in the virus, is due to the action of the protease encoded within the gag-pol gene (Kramer et al. Science 231, 1580 (1986).
A number of mammalian ABC transporters have been expressed in yeast to determine their ability to substitute for yeast Ste6p in the transport of pheromone. The mammalian proteins thus far tested include human Mdr1 (Kuchler and Thorner, Proc. Natl. Acad. Sci. 89, 2302 (1992)) and murine Mdr3 (Raymond et al. Science 256, 232 (1992), proteins involved in multidrug resistance; in addition, a chimeric protein containing human CFTR (cystic fibrosis transmembrane conductance regulator) and yeast STE6 sequence has been shown to transport a-factor pheromone in yeast (Teem et al. Cell 73, 335 (1993).
An a cell may be engineered to produce the a-factor receptor, and an α cell to make α-factor receptor, Nakayama, et al., EMBO J., 6:249-54 (1987); Bender and Sprague, Jr., Genetics 121: 463-76 (1989).
Heterologous G protein-coupled receptors have been functionally expressed in S. cerevisiae, Marsh and Hershkowitz, Cold Spring Harbor Symp., Quant. Biol., 53: 557-65 (1988) replaced the S. cerevisiae STE2 with its homologue from S. Kluyven. More dramatically, a mammalian beta-adrenergic receptor and Gα subunit have been expressed in yeast and found to control the yeast mating signal pathway, King, et al., Science, 250: 121-123 (1990).
Duke University. WO92/05244 (Apr. 2, 1992) describes a transformed yeast cell which is incapable of producing a yeast G protein α subunit, but which has been engineered to produce both a mammalian G protein α subunit and a mammalian receptor which is “coupled to” (i.e., interacts with) the aforementioned mammalian G protein α subunit. Specifically, Duke reports expression of the human beta-2 adrenergic receptor (hβAR), a seven transmembrane receptor (STR), in yeast, under control of the GAL1 promoter, with the hβAR gene modified by replacing the first 63 base pairs of coding sequence with 11 base pairs of noncoding and 42 base pairs of coding sequence from the STE2 gene. (STE2 encodes the yeast α-factor receptor). Duke found that the modified hβAR was functionally integrated into the membrane, as shown by studies of the ability of isolated membranes to interact properly with various known agonists and antagonists of hβAR. The ligand binding affinity for yeast-expressed hβAR was said to be nearly identical to that observed for naturally produced hβAR.
Duke co-expressed a rat G protein α subunit in the same cells, yeast strain 8C, which lacks the cognate yeast protein. Ligand binding resulted in G protein-mediated signal transduction.
Duke teaches that these cells may be used in screening compounds for the ability to affect the rate of dissociation of Gα from Gβγ in a cell. For this purpose, the cell further contains a pheromone-responsive promoter (e.g. BAR1 or FUS1), linked to an indicator gene (e.g. HIS3 or LacZ). The cells are placed in multi-titer plates, and different compounds are placed in each well. The colonies are then scored for expression of the indicator gene.
Duke's yeast cells do not, however, actually produce the compounds to be screened. As a result, only a relatively small number of compounds can be screened, since the scientist must ensure that a given group of cells is contacted with only a single, known compound.
Yeast have been engineered to express foreign polypeptide variants to be tested as potential antagonists of mammalian receptors. Libraries encoding mutant glucagon molecules were generated through random misincorporation of nucleotides during synthesis of oligonucleotides containing the coding sequence of mammalian glucagon. These libraries were expressed in yeast and culture broths from transformed cells were used in testing for antagonist activity on glucagon receptors present in rat hepatocyte membranes (Smith et al. 1993).
Drugs which overcome the multiple drug resistance (MDR) of cancer cells may be identified by using transformed yeast cells expressing P-glycoprotein (Suntory Ltd., patent application JP 2257873 entitled “Multiple drug resistance-relating gene—comprises P-glycoprotein accumulated in cell membrane part of transformed yeast”). The drugs were not produced by the yeast cells in question.
A yeast strain has been derived to allow the identification of inhibitors of protein farnesyltransferase which exhibit activity against mammalian Ras and which may therefore function as antitumor drugs (Hara et al. 1993).
Genetic Markers in Yeast Strains
Yeast strains that are auxotrophic for histidine (HIS3) are known, see Struhl and Hill), Mol. Cell. Biol., 7:104 (1987); Fasullo and Davis, Mol. Cell. Biol., 8:4370 (1988). The HIS3 (imidazoleglycerol phosphate dehydratase) gene has been used as a selective marker in yeast. See Sikorski and Heiter, Genetics, 122:19 (1989); Struhl, et al., P.N.A.S. 76:1035 (1979); and, for FUS1-HIS3 fusions, see Stevenson, et al., Genes Dev., 6:1293 (1992).
Peptide Libraries. Peptide libraries are systems which simultaneously display, in a form which permits interaction with a target, a highly diverse and numerous collection of peptides. These peptides may be presented in solution (Houghten), or on beads (Lam), chips (Fodor), bacteria (Ladner U.S. Pat. No. 5,223,409), spores (Ladner USP '409), plasmids (Cull) or on phage (Scott, Devlin, Cwirla, Felici, Ladner '409). Many of these systems are limited in terms of the maximum length of the peptide or the composition of the peptide (e.g., Cys excluded). Steric factors, such as the proximity of a support, may interfere with binding. Usually, the screening is for binding in vitro to an artificially presented target, not for activation or inhibition of a cellular signal transduction pathway in a living cell. While a cell surface receptor may be used as a target, the screening will not reveal whether the binding of the peptide caused an allosteric change in the conformation of the receptor.
Ladner, U.S. Pat. No. 5,096,815 describes a method of identifying novel proteins or polypeptides with a desired DNA binding activity. Semi-random (“variegated”) DNA encoding a large number of different potential binding proteins is introduced, in expressible form, into suitable host cells. The target DNA sequence is incorporated into a genetically engineered operon such that the binding of the protein or polypeptide will present expression of a gene product that is deleterious to the gene under selective conditions. Cells which survive the selective conditions are thus cells which express a protein which binds the target DNA. While it is taught that yeast cells may be used for testing, bacterial cells are preferred. The interactions between the protein and the target DNA occur only in the cell (and then only in the nucleus), not in the periplasm or cytoplasm, and the target is a nucleic acid, and not a pheromone system protein surrogate.
Substitution of random peptide sequences for functional domains in cellular proteins permits some determination of the specific sequence requirements for the accomplishment of function. Though the details of the recognition phenomena which operate in the localization of proteins within cells remain largely unknown, the constraints on sequence variation of mitochondrial targeting sequences and protein secretion signal sequences have been elucidated using random peptides (Lemire et al., J. Biol. Chem. 264, 20206 (1989) and Kaiser et al. Science 235, 312 (1987), respectively).
All references cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art.